Photoabsorption Measurements of Atomic and Molecular Potassium

4 Photoabsorption Measurements of Atomic and Molecular Potassium C. Y. R. WU

Downloaded by CORNELL UNIV on October 10, 2016 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch004

Department of Physics and Earth and Space Sciences Institute, University of Southern California, Los Angeles, C A 90007

Photoabsorption measurements o f atomic potassium in the 50-70 nm region and molecular potassium i n the 210-400 nm region have been c a r r i e d out by using a conventional heat-pipe oven. An experimental proce dure which has been proven to be e f f e c t i v e i n reducing the experimental u n c e r t a i n t y is d e s c r i b e d . The trend of absorption cross s e c t i o n s , i . e . , i n c r e a s i n g as the photon energy i n c r e a s e s in the 50-70 nm r e g i o n , sup ports t h e o r e t i c a l r e s u l t s which i n d i c a t e that the c o r r e l a t i o n e f f e c t s between the 4s and 3p e l e c t r o n s are important though the c o r r e l a t i o n e f f e c t s may not be strong enough to significantly a l t e r the t o t a l oscillator s t r e n g t h o f every i n d i v i d u a l orbital e l e c tron. No v i b r a t i o n a l s t r u c t u r e a s s o c i a t e d with the p h o t o i o n i z a t i o n o f molecular potassium i n the UV region is observed. Further experiments utilizing different technique are i n progress. Recently i n our l a b o r a t o r y we have i n i t i a t e d a program to study the photoabsorption processes o f metal vapors throughout the UV and EUV region. Our research i n t e r e s t s are (1) to o b t a i n the absolute cross s e c t i o n measurement o f atomic and molecular metal vapors, (2) to study the p h o t o i o n i z a t i o n processes o f molecular metal s p e c i e s , and (3) to study the p h o t o d i s s o c i a t i o n processes o f molecular metal i o n s . S e v e r a l experimental methods such as the heat-pipe absorption spectroscopy, p h o t o i o n i z a t i o n mass spectroscopy, and e l e c t r o n - i o n coincidence technique, w i l l be used i n the study. This report summarizes our f i r s t experiment using heat-pipe absorption spectroscopy. Experimental The photoabsorption of atomic and molecular potassium were measured i n the extreme u l t r a v i o l e t (EUV, 50-70 nm) and u l t r a v i o l e t (UV, 210-400 nm) region. In the f i r s t experiment the 0097-6156/82/0179-0043$05.00/0

©

1982 American Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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METAL BONDING AND INTERACTIONS

atomic l i n e emissions of Ar, 0, and N ions were produced i n a r e p e t i t i v e spark discharge and were used as the l i g h t source. An AJ£ metal t h i n - f i l m was used (1) to separate the heat-pipe absorpt i o n c e l l from the vacuum monochromator and (2) as an o p t i c a l window with t r a n s m i s s i o n i n the 17-76 nm region. In the second experiment, using a 1KW Hg a r c lamp as the l i g h t source, the absorption i n the 210-400 nm region was measured. A quartz window was used to separate the heat-pipe c e l l and the monochromator. A heat-pipe tube (JL) , 4 feet long and 3-1/3 inches i n diame t e r , i s made of 304 s t a i n l e s s s t e e l and has a w a l l t h i c k n e s s of 65 m i l s . The temperature p r o f i l e of the heat-pipe oven i s measured by 21 thermocouples attached to the o u t s i d e s u r f a c e of the heat^pipe and by 8 movable thermocouples i n s i d e the heat-pipe. The heat-pipe was t y p i c a l l y operated at a pressure of 0.1 T o r r with a 50 cm long i s o t h e r m a l zone. Figure 1 shows a measured temperature p r o f i l e . Since the d i r e c t l y measured temperature at the center region i s h i g h , caused by d i r e c t exposure of the thermocouples to the heater, the temperature i n t h i s region (dotted l i n e ) i s i n t e r p o l a t e d from the temperature readings at both ends. A constant temperature f o r the metal at the zone i s assumed. T h i s i s confirmed by n e a r l y constant temperature readings obtained with the movable thermocouples i n s i d e the heat-pipe. When the heat-pipe i s i n a steady s t a t e the pressure of the metal vapor i s equal to that' of the b u f f e r gas. The vapor p r e s sure i n the isothermal zone i s thus determined by measuring the b u f f e r gas pressure with an MKS Baratron capacitance manometer. The photoabsorption cross s e c t i o n i s measured by the s p l i t beam technique, i . e . , by simultaneously monitoring the l i g h t i n t e n s i t i e s both before and a f t e r the photon beam passes through the heat-pipe oven. The spectrometer bandwidth was set at 0.08 nm i n the present measurement. Data Reduction The main d i f f i c u l t i e s i n the absolute photoabsorption c r o s s s e c t i o n measurements are the determination of the number d e n s i t i e s of the metal vapors, the presence of metal c l u s t e r s , and the outgassing problem. The number d e n s i t y can now be f a i r l y a c c u r a t e l y determined by using the heat-pipe technology. The presence of a small percentage of metal dimer or other outgassing products could lead to a n o t i c e a b l e u n c e r t a i n t y i n the absolute atomic cross s e c t i o n determination s i n c e the molecular c r o s s s e c t i o n can be as high as 10-100 times l a r g e r than that of atom ( 2 ) . To m i n i mize such u n c e r t a i n t y , we f i n d that i t i s necessary to operate the heat-pipe oven at the lowest temperature p o s s i b l e V500°K (0.1 T o r r He b u f f e r gas p r e s s u r e ) , as we have done i n the present work. To f u r t h e r reduce the a b s o r p t i o n due to the r e s i d u a l outgassing products a s p e c i f i c experimental procedure has been c a r r i e d out i n our measurements. Namely, a f t e r r e c o r d i n g the


Atomic

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current r a t i o , ( i / i ) , of the i n c i d e n t and the transmitted l i g h t i n t e n s i t i e s and the temperature and pressure readings of the heatp i p e , we r a p i d l y c o o l the heat-pipe to near room temperature ( t y p i c a l l y i n ^ 20 minutes). By t h i s time, the metal vapor pressure i s reduced to p r a c t i c a l l y zero. The current r a t i o , ( i / i o ) f , i s again measured and assumed to contain a l l the outgassing products. The photoabsorption cross s e c t i o n i s thus given by 0

a = [ln(i/i ) 0

f

- ln(i/io)]/[n

0

(

(P(x)/T(x))dx]

where n = 9.6586 i s a constant, P(x) i s the pressure i n T o r r , T(x) i s the temperature i n °K, and L i s the absorption path l e n g t h of metal vapors.


0

Results and

Discussion

The EUV Region. The r e s u l t s f o r the photoabsorption cross s e c t i o n s of potassium vapor i n the energy region 50.8 nm - 76.0 nm are shown i n Figure 2. The e l e c t r o n i c c o n f i g u r a t i o n of potassium i n i t s ground s t a t e i s KK 3 s 3 p 4 s and the thresholds f o r i o n i z i n g a 4s and a 3p e l e c t r o n are at 285.7 nm (4.339 eV) and 48.1 nm (25.74 eV), respectively. From the sum r u l e and the independent s u b s h e l l assumption the o s c i l l a t o r s t r e n g t h from 3p w i l l be a f a c t o r of 6 over that of 4s. In other words, the i n t e g r a t e d p h o t o i o n i z a t i o n cross s e c t i o n f o r the 3p e l e c t r o n w i l l be a f a c t o r of s i x l a r g e r than that of 4s i f there i s no e l e c t r o n c o r r e l a t i o n between 3p and 4s e l e c t r o n s , or e l e c t r o n s of other s u b s h e l l s . The c a l c u l a t e d cross s e c t i o n s (3) based on a s i n g l e - e l e c t r o n theory and using the atomic wavefunctions computed with Herman and Skillman p o t e n t i a l s (_4) are a l s o i n d i c a t e d i n Figure 2. Hudson and C a r t e r (5) and Marr and Creek (6) measured the p h o t o i o n i z a t i o n cross s e c t i o n from the t h r e s h o l d of the 4s i o n i z a t i o n p o t e n t i a l to 68.0 nm and to 124.0 nm, r e s p e c t i v e l y . D r i v e r (3) measured the cross s e c t i o n from 50.0 to 35.0 nm, i . e . , the 3p continuum. The cross s e c t i o n s of some of the a u t o i o n i z i n g l i n e s between 65.0 - 50.0 nm were a l s o measured (4,5). Our data mainly cover the gap between that of Hudson and C a r t e r (5) and D r i v e r (3). In the region where our data overlap with those of Hudson and C a r t e r , the r e s u l t s are i n reasonable agreement, although our data are c o n s i s t e n t l y higher than those of Hudson and C a r t e r . The reason may be due to the d i f f e r e n t experimental techniques employed. Our measured data show that the photoabsorption cross sect i o n of potassium i n c r e a s e s monotonically as photon energy i n creases. This i s expected from the 4s - 3p e l e c t r o n i c c o r r e l a t i o n e f f e c t as discussed by Chang ( 7 ) . Without such a c o r r e l a t i o n e f f e c t the cross s e c t i o n should decrease toward s h o r t e r wavelengths as p r e d i c t e d by the s i n g l e - e l e c t r o n theory. 2

6


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PHOTON ENERGY(eV) 20 30

40

50 40 WAVELENGTH (nm) Figure 2. Summary of experimental cross sections for atomic K. Key: ·, present work; O, Ref. 3; , Ref. 5; · · ·, Ref. 6. The calculated result ( ) based on a single-electron model is also shown for comparison (3).

Figure 3.

The cross sections of Κ molecule in the 210-400 nm region.




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The data p o i n t at 50.8 nm measured from the present e x p e r i ment seems to suggest that the cross s e c t i o n of the 4s continuum w i l l j o i n the 3p continuum with a sharp step i n c r e a s e . T h i s may support the argument made by D r i v e r that the c o r r e l a t i o n e f f e c t between 4s and 3p e l e c t r o n s i s not strong enough to i n c r e a s e the sum of the o s c i l l a t o r s t r e n g t h of the 4s e l e c t r o n s i g n i f a n t l y beyond the expected value of one. The unusually l a r g e cross s e c t i o n measured at 55.5 nm may r e s u l t from the absorption by one of the higher members of the autoionizing l i n e s . The l i n e shapes and cross s e c t i o n s of these a u t o i o n i z i n g l i n e s are d i f f i c u l t to determine u s i n g the l i n e source such as the one used i n the present experiment. A synchrotron r a d i a t i o n source may provide an i d e a l continuum l i g h t source f o r such d e t a i l e d s t u d i e s . The UV Region. The r e s u l t s f o r the r e l a t i v e cross s e c t i o n s of potassium vapor i n the energy region of 210-400 nm are shown i n Figure 3. Because of strong absorptions i n t h i s wavelength region the vapor pressure of potassium i n the heat-pipe could not be reduced to the p o i n t such that the heat-pipe operation condit i o n could s t i l l be s u s t a i n e d . The isothermal zone i s thus not established. Therefore, only r e l a t i v e data are measured i n the present work. Hudson and C a r t e r (8) and Creek and Marr (_2) measured the c r o s s s e c t i o n of the potassium molecule up to 300 and 350 nm, respectively. The shapes of t h e i r absorption curves do not agree with our r e s u l t s . A constant a b s o r p t i o n i n the ^ 230-290 nm r e g i o n was observed by the previous i n v e s t i g a t o r s (_2,8) while a sharp increase i n absorption s t a r t i n g from ^ 230 nm can be seen i n Figure 3. The o r i g i n of t h i s discrepancy i s not known. Recent experimental and t h e o r e t i c a l r e s u l t s show the d i s s o c i a t i o n energies f o r the ground s t a t e s of L±2 (9,10) , Na£ (11,12, 13), and K (12,13) are 10324, 7920, and 6404 cm" , r e s p e c t i v e l y . Thus, one would expect to see some v i b r a t i o n a l s t r u c t u r e s a s s o c i ated with the p h o t o i o n i z a t i o n t r a n s i t i o n . In f a c t , s e v e r a l autoi o n i z i n g Rydberg s t a t e s of K have been observed i n a s e q u e n t i a l two-photon i o n i z a t i o n technique (14). However, i n the t o t a l absorption p l o t no s t r u c t u r e i s observed on the high energy s i d e of the i o n i z a t i o n t h r e s h o l d of K2 as shown i n Figure 3. A possible explanation i s that the p h o t o i o n i z a t i o n cross s e c t i o n s are smaller than those of other processes, e.g., Rydberg and p h o t o d i s s o c i a t i v e s t a t e s of the n e u t r a l K2 molecule, such that any s t r u c t u r e assoc i a t e d with the p h o t o i o n i z a t i o n t r a n s i t i o n may be b u r i e d i n the t o t a l absorption cross s e c t i o n s . A study of t h i s problem using the p h o t o i o n i z a t i o n mass spectrometry i n a one-photon e x c i t a t i o n process i s c u r r e n t l y i n progress i n our l a b o r a t o r y . +

1

2

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Atomic and Molecular Potassium

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Acknowledgements The author wishes to express his gratitude to Drs. T. N. Chang, D. L. Judge, C. C. Kim, and N. Shen for helpful discussions and assistance in various stages of this work. Literature Cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Vidal, C. R.; Cooper, J. J . Appl. Phys. 1969, 40, 3370-4. Creek, D. M.; Marr, G. V. J. Quant. Spectrosc. Radiat. Transfer. 1968, 8, 1431-6. Driver, R. D. J. Phys. B. 1976, 9, 817-27. Herman, F . ; Skillman, S. "Atomic Structure Calculations"; Englewood Cliffs, N . J . , Prentice-Hall, 1963. Hudson, R. D.; Carter, V. L. J. Opt. Soc. Am. 1967, 57, 1471-4. Marr, G. V.; Creek, D. M. Proc. Roy. Soc. A. 1968, 304, 233-44. Chang, T. N. J . Phys. B. 1975, 8, 743-50. Hudson, R. D.; Carter, V. L. Phys. Rev. 1965, 139, A1426-8. Mathur, B. P.; Rothe, E. W.; Reck, G. P.; Lightman, A. J . Chem. Phys. Letters, 1978, 56, 336-8. Konowalow, D. D.; Rosenkrantz, M. E. Chem. Phys. Letters. 1979, 61, 489-94. Bardsley, J . N.; Junker, B. R.; Norcross, D. W. Chem. Phys. Letters, 1976, 37, 502-6. Leutwyler, S.; Hofmann, M.; Harri, H-P.; Schumacher, E. Chem. Phys. Letters. 1981, 77, 257-60. Habitz, P.; Schwartz, W. H. E. Chem. Phys. Letters, 1975, 34, 248-52. Leutwyler, S.; Herrmann, A.; Woste, L.; Schumacher, E. Chem. Phys. 1980, 48, 253-67.

RECEIVED August 26, 1981.


Photoabsorption Measurements of Atomic and Molecular Potassium

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